Presently, both military and civilian users of electronic equipment require the use of rechargeable lithium-ion batteries to provide power to numerous portable electronic devices such as cellular telephones, laptop computers, communication systems and digital instruments at an ever-increasing pace. Lithium-ion batteries provide a much-needed source of power for portable electrically operated devices, but they also pose a number of drawbacks and problems.
The typical lithium-ion battery is composed of an anode, or negative electrode, of graphite and a cathode, or positive electrode, usually made from a transition metal oxide or sulfide compound. Rechargeable lithium-ion batteries typically use electrolytes containing a lithium salt dissolved in an organic solvent. Prior art mixed organic solvent electrolytes were developed using lithium salts dissolved in mixtures of a high dielectric constant solvents, such as Ethylene Carbonate (“EC”) and Propylene Carbonate (“PC”), with various volume ratios of low viscosity ester solvents such as Diethyl Carbonate (“DC”) and dimethyl carbonate (“DMC”). The lithium salt of choice for these electrolyte solutions has been Lithium Hexafluorophosphate, LiPF6, because LiPF6 provides a high specific ionic conductivity and possesses wide electrochemical stability over the high operating voltage range of lithium-ion cells of about 2.5 V to 4.3 V as compared with lithium, but LiPF6 suffers from a number of disadvantages, shortcomings and limitations.
One significant problem with LiPF6 in the electrolyte solution is that it reacts with trace water present in the cells to produce hydrofluoric acid and other reaction products that react with the solvents to produce more water, resulting in an overall continued decomposition of the salt. Another serious limitation with LiPF6 in the electrolyte is that it thermally decomposes at moderately low temperatures of about 65° C. and the reaction is accelerated at higher temperatures and even more rapid at temperatures above 65° C. The result is a significant loss of battery performance due to reduced cell capacity and poor cycle life. These two drawbacks of reactivity with trace water and low temperature thermal decomposition significantly limit the use and performance of lithium-ion cells and batteries containing LiPF6 in higher temperature environments. Thus there has been a long-felt need for LiPF6 lithium-ion cells and batteries that do not decompose in the presence of water and have a higher temperature of stability, thereby eliminating the hydrofluoric acid reaction products that typically degrade cell performance, without suffering from the disadvantages, shortcomings and limitations of prior art electrochemical systems.
Up until now there has been no LiPF6 salt that does not suffer from the disadvantages, limitations, drawbacks and shortcomings of LiPF6 electrolytes for lithium-ion cells and batteries of trace water reactivity and low temperature thermal decomposition. The present inventors have discovered a new phosphate free, borate lithium salt electrolyte composed of Lithium tetra-phenyl-3,5 trifluoromethyl borate, having the formula LiBC32F24H12, hereinafter referred to as LiTPTB, in a ternary mixed organic solvent containing a 1:1:1 volume ratio of EC, DMC, and EMC that provides substantially improved cell performance for use in electrolytes for lithium-ion cells and batteries.